"John Ings" wrote

1. If it's thrust against the front compressor blades, how did engines
with centrifugal compressors ever get an aeroplane off the ground?


Focus on the combustion chamber, and what goes on in it. There's high pressure
in the combustion chamber. There's a hole in the front for air to come in, and
a hole in the back for air to go out. The pressure of the air into, within, and
exiting the chamber is constant. What changes is its _temperature_, and
therefore its volume.

So you take a compressor of any kind -- even a piston compressor. You pump air
up to combustion chamber pressure with it. (If you don't pump it up to at least
that pressure, it won't go into the chamber.) The air you're pumping is cold,
so its volume is small, per unit mass. So you can shove it into the chamber
through a fairly small opening, at some velocity. In the chamber, you heat it
like hell, but at constant pressure. (It heats up at constant pressure because
you're allowing it to expand in volume.) The hot, expanded air leaves the back
end of the chamber.

We assume, for simplicity, that the air's velocity is the same going out the
chamber as it was coming in. Now, the _mass_ flow of air through the chamber is
constant. If you have a larger _volume_ flow out the back, because the air is
hotter, then you need a bigger hole in the back than you had in the front. So
look what you have: a pressurized chamber with a small hole in front, and a
large hole in back. Which way will it want to move?

Bottom line: the thrust comes from pressurized air pushing on the bigger area
at the front of the chamber. The front area is bigger because the front hole is
smaller. You get to pull off this neat trick because you burn lots of fuel to
_heat_ the air.

In a more realistic (but still cartoonish) jet engine, where the compressor in
front and the turbine in back are on the same spindle, you'll note that the
compressor pulls forward on the spindle, but the turbine pulls backward. The
_net_ force on the spindle bearings is the difference between them, and I bet
it's small, in practical engines.

-- Tony P.